Scala 多繼承問題

多繼承問題：

 

object LoadIssueDemo extends App {

  import java.io.PrintWriter

  trait Logger {
    def log(msg: String): Unit
  }

  trait FileLogger extends Logger {
    val fileName: String
    val fileOutput = new PrintWriter(fileName: String)
    fileOutput.println("#")

    def log(msg: String): Unit = {
      fileOutput.print(msg)
      fileOutput.flush()
    }
  }

  class Person

  class Student(var name: String) extends Person with FileLogger {
    override val fileName: String = "file.log"

  }

  new Student("Win").log("trait demo")

}

運行結果：

 

原因：fileName還未初始化，就被方法調用。

解決方法：

1. 提前定義 （代碼不夠優雅）

object PreDefineDemo  extends App{
  import java.io.PrintWriter

  trait Logger{
    def log(msg: String): Unit
  }

  trait FileLogger extends Logger {
    val fileName: String
    val fileOutput = new PrintWriter(fileName: String)
    fileOutput.println("#")

    def log(msg: String): Unit ={
      fileOutput.print(msg)
      fileOutput.flush()
    }
  }

  class Person
  class Student(var name: String) extends Person with FileLogger{
    override val fileName: String = "file.log"

  }

  new { override val fileName: String = "file.log"} with Student("Win").log("trait demo")

}

2. 懶加載（推薦）

object LazyLoadDemo extends App{
  import java.io.PrintWriter

  trait Logger{
    def log(msg: String): Unit
  }

  trait FileLogger extends Logger {
    val fileName: String
    lazy val fileOutput = new PrintWriter(fileName: String)

    def log(msg: String): Unit ={
      fileOutput.print(msg)
      fileOutput.flush()
    }
  }

  class Person
  class Student(var name: String) extends Person with FileLogger{
    override val fileName: String = "file.log"

  }

  val s = new Student("Win")
  s.log("#")
  s.log("Lazy demo")

}